It's obviously a �. Or perhaps a □ . Maybe an ¾, on odd Wednesdays?

(Unicode has its strengths. Making up replacement characters isn't one.)

I happen to know that the biggest ski resort reservation system in Scandinavia contains a function called MaybeOnATuesday(), but to my knowledge it's never called.

Yeah...what I'd really like to do would be to give a character a "natural" background color, e.g.

Then its simple for support to say "red is one, green is ell". But you can't just add a color to a character, because copy paste/rich formatting don't work everywhere, or even transfer well...

Alternatively, if you use ⓪ and ⒈ it matters less if the user says "at" or "zero", and more that they didn't say "oh" or "one".